1. Field of the Invention
The present invention relates to an apparatus for forming an image by scanning the light beam.
2. Related Background Art
FIG. 1 is a cross-sectional view for explaining the constitution of such an image forming apparatus, wherein 1 is a photosensitive drum which is an electrostatic latent image carrier, 2 is a semiconductor laser which is a light source, 3 is a polygon scanner for scanning the light beam, 4 is an optical lens, 5 is a charger roller for uniformly charging the photosensitive drum 1, 6 is a developer for developing an electrostatic latent image formed on the photosensitive drum 1 with a toner, 7 is a transfer roller for transferring a toner image formed on the photosensitive drum 1 onto a paper of recording medium, 8 is a fixer roller for fixing the toner transferred onto the paper, 9 is a paper cassette for stacking papers, 10 is a paper feed roller for feeding a paper within the paper cassette 9 into a paper conveying passage, 11 is a paper detecting sensor (resist sensor) for detecting the paper fed therein, 12 is a resist roller for synchronizing the application of image data onto the photosensitive drum 1 with the feeding of paper, as well as correcting for an oblique movement of paper because the paper will abut once against the roller, and 13 is a paper discharge sensor for sensing the paper discharged.
FIG. 2 is a block diagram for explaining one example of a control system of the image forming apparatus as shown in FIG. 1, wherein 14 is a host computer which sends out code data of image to a print controller 15. The print controller 15 monitors the state of a printer engine or directs the operation condition, as well as expanding the code data of image into a predetermined bit map data. A print control unit 16 is to control each mechanism of the printer engine based on instructions of the printer controller 15, each mechanism sending out necessary data to the print control unit 16 and operating with instructions of the print control unit 16. The mechanism is composed of a paper feed driver 17, a high voltage driver 18, a scanner motor driver 19, and a laser driver 20. The paper feed driver 17 supplies the information of sensors 22 such as a paper detecting sensor to the print control unit 16, as well as driving or stopping rollers 21 based on instructions of the print control unit 16. The high voltage driver 18 drives or stops a heater in a fixer 26, as well as drives or stops each high voltage of a charger 23, a developer 24, and a transfer unit 25 with instructions of the print control unit 16. And it returns the temperature information of the heater necessary for controlling the print control unit 16. The scanner motor driver 19 returns the information of whether or not the scanner motor 27 reaches a predetermined rotation number, as well as drives or stops the scanner motor 27 with the instruction of the print control unit 16. 28 is a laser driver for turning on or off a semiconductor laser, not shown, based on the print data. 29 is a laser adjuster for detecting the quantity of a laser light emitted and returning a detected output to the laser driver 20.
Referring now to FIGS. 3 and 4, a print sequence in a conventional image forming apparatus will De described below.
FIG. 3 is a flowchart showing one example of a printing procedure in the image forming apparatus as shown in FIG. 1. Note that (1) to (17) show each step.
If the power is turned on, its own memory such as RAM or each mechanism is first initialized (1). Next, the fixer heater is warmed up at a predetermined temperature (2). If the warming up has been completed, it is placed in a state of receiving a print request from the print controller 15. If a print request signal is received (4), the drive of scanner motor 27 (5) is started by issuing an instruction for starting the drive of scanner motor 27, and the drive of carrier motor is started by issuing an instruction for starting the drive of carrier motor (6). Then, the high voltage driver 18 directs the set-up of each high voltage of charger 23, developer 24 and transfer unit 25 (7). Next, a determination is made whether or not the 10 scanner motor 27 has been set up at a predetermined number of rotations (8), in which if the number of rotations reaches a predetermined one and a scanner ready is received from the scanner motor driver 19, the adjustment of laser light quantity is started to set up at a predetermined value (9). If the light quantity reaches a predetermined level (10), an instruction for feeding a paper from the paper cassette 9 is issued (11). Then the procedure waits for the paper to reach a disposed position of resist sensor 11 (12), and if the paper reaches the disposed position of resist sensor 11, the procedure waits for a predetermined period of T1 sec to correct for the oblique movement (13). And the paper feed roller 10 is stopped, and a vertical synchronizing request signal that is an image output synchronizing signal is issued to the print controller 15 (14). If the vertical synchronizing signal is received from the print controller 15 (15), the print is started by applying the laser beam onto the photosensitive drum 1 (16). Next, if the paper discharge sensor 13 detects a paper discharged (17), one page print is terminated by stopping each driver.
FIG. 4 is a timing chart for explaining one example of the print sequence in the image forming apparatus as shown in FIG. 1. Note that t1 to t7 indicate each timing.
If the power is turned on (t1), the heater temperature of fixer roller 8 is set up at a predetermined one (t2). Then if a print request signal is received (t3), the operation is placed in a scanner ready state (t4), so that the pick up of a paper is started (t5). Then, if the vertical synchronizing signal is received (t6), the print processing is started, and if the paper discharge sensor 13 detects a paper discharged (t7), one page print is terminated by stopping each driver.
However, the above conventional example had a problem that the time from a standby state via the reception of a print request to the print termination (referred to as a first print time) was long, because the carrier motor and the scanner motor 27 were started (t3) after the reception of the print request, and further the laser light quantity adjustment was started after waiting for the scanner to reach a predetermined rotation number (t4).
Also, this type of recording apparatus first performed the print preparatory operation if it receives a print start request signal (print request signal PRINT), and subsequently performed the print operation. Particularly, a laser beam printer (LBP) was constituted in the following way. An image signal transmit control unit (controller) transmits a print signal to enable the LBP to perform the print operation. And if it receives an image signal transmit request signal (vertical synchronizing request signal VSNREQ) from the LBP, it outputs an image transmit synchronizing signal (vertical synchronizing signal VSYNC), and transmits an image signal in synchronism with a main scan synchronizing signal (main scan synchronizing signal BD) for forming the image which is received from the LBP.
On the other hand, if the LBP receives the print request signal, it starts the print operation at the timing not affecting a print preparatory operation, as well as starting the print preparatory operation. That is, the rotation control (forward rotation control) of the photosensitive member (photosensitive drum) is performed for the purpose to place the photosensitive member into an electrostatically stable state by applying a high voltage as the print preparatory operation, light quantity adjusting means (laser APC) is operated for setting the laser light source at a predetermined light quantity, or a polygon mirror scanning motor (scanner motor) which is a light scanning device within a scan optical system is set up at a desired scan speed to cause the light source to be scanned at a certain scan speed. Besides the print preparatory operation, a print paper is fed at a desired timing in accordance with the end timing of print preparatory operation for the print operation. Near the end timing of print preparatory operation, the vertical synchronizing request signal VSNREQ and the vertical synchronizing signal VSYNC are transmitted and/or received, and an image developed on the photosensitive member is synchronized with the print paper (resist adjustment).
Here, the specific timings in the LBP will be described with reference to FIGS. 5A & 5B.
FIGS. 5A & 5B are timing charts for explaining a resist adjusting processing in this kind of recording apparatus, with the axis of ordinates showing the rotation number (rpm) of the scanner motor and the axis of abscissas showing the time (sec).
In FIGS. 5A & 5B, if a print request signal PRINT is received, the prerotation control and the setup of the scanner motor at a set rotation number are first performed. Then at the timing when both operations are completed, the set-up of the laser light quantity is performed at a desired light quantity. At this adjustment timing for the light quantity, firstly, the laser light set-up must be necessarily performed when the laser is scanned by the rotation of the scanner motor, because if the light beam is concentrated at a point on the surface of photosensitive member when the light quantity is applied, the photosensitive characteristics at its portion are significantly degraded so that the image quality may be decreased. Secondly, if the light is applied with the laser light set-up, the photosensitive drum may be exposed to the light, and developed with the developer unit if left away, so that a developing powder may stick onto a transfer roller, making dirty the transfer roller, and thus a back face of print paper, which may significantly degrade the print quality, whereby the developing action must be prevented by turning off the development bias at the timing when an exposed drum face is brought to the developer unit (this processing is referred to as a postprocessing for laser light quantity adjustment). But since the development bias could not be turned off in the prerotation control, it was necessary to make different the phase between prerotation control period, laser light quantity step and processing control period. As above described, the print preparatory operation was started at the timing when the scanner motor was rotating and the postprocessing control for the laser light quantity adjustment did not overlap the prerotation control. Accordingly, the paper feed timing of a print paper was started at a timing 3.3 sec before the end timing of print preparatory operation (a timing after the passage of 5.5 sec from the reception of print request signal PRINT) as shown in FIGS. 5A & 5B.
However, with the constitution of the above constitutional example, if the time from the reception of print signal PRINT to the discharge of print paper (first print time) was calculated, it would be extended by the amount of print preparatory operation (about 2.2 sec) beyond an actual paper conveying time of this LBP.
Accordingly, there was a problem with the LBP which was associated with more intermittent print operations than continuous print operations, that the life of photosensitive member might be decreased, because the rotation of photosensitive member would be increased in proportion to the number of prints (it is said that the life of photosensitive member is proportional to its rotation time).
As shown in FIGS. 5A & 5B, the most consuming time in the print preparatory operation is a required time for setting up the rotation of scanner motor at a set scanning speed, and the time for which the scan speed is judged to have been stabilized. In the LBP, it is set at 3.2 sec by ability. That is, the most important factor in determining the print preparatory operation is a time for setting up the scanner motor rotation at a set scan speed, and a scan speed stabilization time for which the scan speed becomes stable. The set up time of scanner motor rotation can be improved by manipulating the motor drive current. However, the time for the stabilization of scan speed depends on the environmental condition at the service or variations of each device, and in practice, the scan speed stabilization time amounting to a predetermined greatest required time (which was often set at 1 sec in the LBP) was needed.
On the other hand, in recent recording apparatuses, the first print time holds a very important position as one item for the comparison of performance of recording apparatus, and may determine the performance of recording apparatus depending on the first print time. That is, in increasingly intensive development competitions, it is of urgent necessity in the present situation to shorten the first print time without sacrificing the performance, reliability or life of recording apparatus.